In a TV receiver having a zoom feature, the incoming composite video signal CVS is sampled and digitized for storage in suitable memory. To produce zoom, a portion of the information stored in the memory is read out in a manner that fills the entire TV screen. To produce a 3-to-1 zoom, for example, each pixel in the designated area of the memory may be repeated three times to stretch out the zoomed portion in the horizontal dimension, and each row of the zoomed portion may be repeated three times to stretch out the picture in the vertical dimension by a factor of 3 to 1 during the memory reading operation.
Typically, the sampling clock signal CLS has a frequency F.sub.CK that is four times the color subcarrier frequency F.sub.SC. Additionally, the clock signal is desirably locked in phase to the color reference burst component of the incoming television signal. This relationship between the color subcarrier signal CSS and the sampling clock signal facilitates chroma/luma separation and chroma demodulation functions in a digital video signal processing system.
In the NTSC standard format, the frequency of the unmodulated color subcarrier signal is established at 3.58 MHz. The horizontal line scanning frequency F.sub.H is established at 2/455 times the color subcarrier frequency (i.e., F.sub.SC =455/2.multidot.F.sub.H). Since the sampling clock frequency F.sub.CK is set at 4 F.sub.SC, there are 910 clock pulses or samples in every horizontal line period.
However, not all video signals which are compatible for use in the NTSC system conform precisely to the NTSC standard format. For example, non-standard signals produced by video cassette recorders have jittering timebase errors due to instabilities in their tape transport mechanisms. The timebase jitter results in varying horizontal line periods in the reproduced signal. This produces a variation in the number of clock pulses or samples developed per horizontal line. The variation in the number of clock pulses causes the phase of the sampling clock relative to the horizontal and vertical synchronizing signals to change from line to line. This phase variation, in turn, causes misalignment of the respective picture elements from successive lines of the TV raster.
In order to align the picture elements before they are written into the memory, the input signal samples are time shifted or skew corrected for the phase differences between the incoming horizontal sync pulses IHSP's and the sampling clock pulses CLP's. Additionally, the signal samples read out from the memory are corrected for skew errors prior to their application to the RGB matrix of the TV receiver, commonly-assigned U.S. Pat. No. 4,638,860, and entitled "TIMING CORRECTION FOR A PICTURE-IN-PICTURE TELEVISION SYSTEM", describes illustrative circuitry for correcting timebase errors in the incoming and outgoing signals in a memory-based video signal processing system.